This invention relates to electromagnetic wave guidance by devices such as waveguides, planar lines and coaxial cables, and more particularly to the transfer of electromagnetic energy among such devices.
A waveguide is formed by a solid dielectric rod or a dielectric filled tubular conductor capable of guiding electromagnetic waves. A planar line for guiding electromagnetic waves generally takes the form of an extended, narrow member of uniform width which is commonly designated as a microstrip line when the strip is insulated from a single ground plane by a dielectric, and is known as an ordinary strip line when the strip is interposed in a dielectric between ground planes. A coaxial cable guides electromagnetic waves between an elongated inner conductor and an outer conductor that is spaced from and encloses the inner conductor.
Many microwave and millimeter wave systems employ waveguides, planar lines and coaxial cables in conjunction with antennas, high-Q (low loss) filters and oscillators, and nonreciprocal components, such as circulators. The signals from such waveguides are often used in hybrid and monolithic integrated circuits, which generally are of planar construction and cannot receive waveguide energy directly. Consequently a transition must be made from one electrical mode, i.e. pattern of electrical wave motion, to another.
For example, if waveguide energy is in the Transverse Electric (TE) mode in a rectangular waveguide, which is a tubular conductor having a rectangular cross-section, the electric field strength has a sinusoidal distribution across the longer cross-sectional dimension of the guide. If this energy is to be used in a monolithic circuit a transition must be made to the Transverse ElectroMagnetic (TEM) mode, where the electromagnetic field pattern is like that of any ordinary transmission line.
A suitable transition can be made from the waveguide to a planar line or coaxial cable. Conversely, if energy is to be received by a waveguide from a planar line or coaxial cable, the transition is made to the waveguide.
Since a coaxial cable has an inner conductor surrounded by a grounded cylinder, which serves as a reference conductor, and a planar line is formed by a flat, elongated conductor mounted above a ground or reference conduction plane, or between ground planes, a planar line approximates a flattened coaxial line which may have a dielectric fill other than air.
When planar lines are used with wave guides, wave energy must be coupled between the planar line and the associated wave guide. Prior art techniques for coupling striplines to wave guides are illustrated in the following U.S. Patents, the disclosures of which are herein incorporated by reference: U.S. Pat. No. 3,483,489 to Dietrich; U.S. Pat. No. 3,579,149 to Ramsey; U.S. Pat. No. 3,732,508 to Ito et al; U.S. Pat. No. 3,755,759 to Cohn; U.S. Pat. No. 3,882,396 to Schneider; U.S. Pat. No. 3,969,691 to Saul; U.S. Pat. No. 4,143,342 to Cain et al and U.S. Pat. No. 4,754,239 to Sedivec.
All of the foregoing references, except Sedivec, provide transformation between the TE and TEM modes relying on coaxial lines, and are not effective at frequencies greater than 40 GigaHerz (GHz) because of the generation of undesirable TE and TM modes as a result of tolerance and size requirements.
While Sedivec provides a suitable wave guide to stripline transition, it requires a tapered wedge that is mounted behind a movable wall within a wave guide. Since the wall is a reflecting panel, it must be moved to a suitable position in order to accomplish the desired transition with a suitable standing wave ratio.
Other transitions are of the probe type as disclosed by T. Q. Hi and Y. Shoe in "Spectral-domain analysis of E-plane waveguide to microstrip transitions", IEEE Trans. Microwave Theory Tech, vol 37 pp 388-392, Feb. 1989 and J. Machac and W. Menzel, "On the design of waveguide to microstrip and waveguide to coplanar line transitions", 23rd European Microwave Conf., 1993 Madrid Spain, pp 615-616. However, probe transitions generally are undesirable because their structures are complex and they are difficult to seal hermetically.
Transition can also be made using an antipodal finline, where wave guidance is along a narrow channel between coplanar conductors, as discussed in L. J. Lavedan, "Design of waveguide to microstrip transitions specially suited to millimeter--wave application", Electron Lett, vol 13, Sept 1977. Once again suitable hermetic sealing is a problem.
Although a ridged waveguide transition can be used of the kind discussed in W. Menzel and A. Klassen, "On the transition from ridge waveguide to microstrip", Proc. 19th European Microwave Conf., 19898, pp. 1265-1269, again there are mechanical complexities and difficulties in achieving a hermetic seal.
The foregoing transitions also have the objection that they are not simple and compact, and easily integrable with planar circuits. The metal structure of Menzel and Klassen, for example, extends to both sides of the planar substrate and the planar substrate has to be cut to a specific form. Hermetic seal is difficult because a split-block is required for the waveguide mounting.
Another waveguide to microstrip transition module is disclosed in U.S. Pat. No. 5,202,648 which issued to J. H. McCandless on Apr. 13, 1993. The module is an assembly of a base connected to a waveguide and a circuit board, with one side of the board mounted on the base. The other side of the circuit board includes a microstrip that has an associated backshort and a metallic cup bonded to the base and circuit board. This configuration is mechanically and electrically complex and does not achieve suitable power transfer.
Still another microstrip to waveguide transition is disclosed in 42 IEEE Transactions on Microwave Theory and Techniques 1842 and 1843, No. 9, September 1994, by Wilfried Grabherr et al. A slot coupled antenna that radiates into a waveguide requires an internal substrate within the waveguide, desirably at a step transition within the waveguide.
Accordingly, it is an object of the invention to achieve an efficient transition among wave guides, planar lines and coaxial cable. Another object is to provide effective transformation between modes at extra high frequencies (EHF).
A further object of the invention is to provide a simpler transition than is commonly provided by transitions of the probe type, or transitions using antipodal fin lines and ridges within waveguides.
Another object is to achieve transitions which provide effective transformation between modes at extra high frequencies (EHF), and yet are wide-banded.
A still further object is to facilitate hermetic sealing when there is a transition between modes of wave guidance. A related object is to avoid the objections that commonly attend probe transitions between planar lines and waveguides.
Still another object is achieve transitions which can cover the full spectrum of microwave to millimeter wave wave guidance. A related object is to achieve suitable transitions from the band of 8.2 to 12.4 GHz, up to 100 GHz.